1. Field of the Invention
The field of art to which this invention pertains is aromatic polyamide fibers and, more particularly, it is directed to a method of printing or overprinting such fibers.
More specifically, in the method of this invention, a diamine salt is imbibed into a fiber structure or tow of never-dried Poly(meta-phenylene isophthalamide) fibers or filaments to provide dye sites for acid dyes after the fibers are dried. The diamine salt, along with a surfactant and, optionally, an ultraviolet light screener and an acid dye are formed into an aqueous mixture and padded out the fibers prior to simultaneous inhibition into the fibers. After drying, the fibers may be printed with acid dyes, without requiring the use of carriers or swelling agents, using conventional printing or overprinting techniques. The diamine salt and surfactant provide effective dye sites for the acid dyes. The overprinted fibers have improved dye shade and more shade depth than do known similar fibers.
2. Description of the Related Art
Aromatic polyamide fibers are known to the art. They have high tensile strength, are flame and heat resistant, possess good flex life, and have high melting points which make them particularly suited to be formed into fabrics usable as protective clothing, and for many other uses.
It further is known that while such aromatic polyamide fibers possess many desired properties as manufactured they also require, for given uses, that various steps be taken to improve a property or properties of the fibers to meet a specific end use. As an example, various additives such as dyes, ultraviolet light screeners, flame retardants, antistatic agents or water repellents, may be incorporated into the fibers during manufacture or such fibers may be treated in subsequent processing steps to improve their performance or appearance levels.
This invention is specifically directed to an improved method of printing or overprinting aromatic polyamide fibers of a poly(meta-phenylene isophthalamide) polymer, hereinafter referred to as "MPD-I fibers". Such fibers, which are described in greater detail in U.S. Pat. No. 3,287,324 to Sweeny, for example, possess many useful properties. It is well known to the art, however, that these fibers are difficult to dye or to print or overprint, using conventional techniques. It further is known that such fibers have little affinity for acid dyes. For this reason water-soluble cationic dyes are generally used exclusively to dye or print these materials.
In certain uses, however, such cationic dyes are unacceptable. As an example, when fabrics made from these fibers are overprinted for military camouflage purposes only certain water-soluble acid dyes, which have an acceptable infrared reflectance, can be used.
Various other problems associated with dyeing or coloring or printing MPD-I fibers, and proffered solutions to these problems, can be seen in U.S. Pat. No. 4,883,496 to Ghorashi and British patent 1,438,067 to Moulds and Vance, the teachings of which are incorporated herein by reference.
The limitations on the type of dye which may be used to dye MPD-I fibers have also been addressed by the art. For example, U.S. Pat. Nos. 3,558,267 to langenfeld; 4,525,168 to Kelly and 4,710,200 to Cates et al. disclose that almost any conventional dyestuff can be used to dye MPD-1 fibers, including acid dyes, by making a solution of the dye in a liquid which is a solvent or strong swelling agent for the fiber, or in a concentrated aqueous solution of the liquid, and heating the fiber in the resulting solution, or by incorporating a dye site substance into the fibers in the presence of a strong polar solvent, followed by the dyeing or printing operation. The problem with these approaches to coloring MPD-I fibers is that the fiber properties are usually adversely affected by the solvent or swelling agent. Also, recovery of the liquid remaining after dyeing or disposing of it in a non-polluting manner is a problem.
Accordingly, a method has long been sought for printing or overprinting MPD-I fibers with acid dyes to obtain improved coloration while retaining good fiber properties. This invention provides such a method by simultaneously imbibing a specific diamine salt (e.g., hexamethylenediamine dihydrochloride), along with a surfactant, into never-dried MPD-I fibers. Preferably, an ultraviolet light screener is imbibed into the never-dried fibers at the same time, along with an acid dye which serves as background coloration in the fibers when when dried, after which such fibers may be readily overprinted with additional acid dyes, using conventional rotary screen printing techniques.
The surfactant serves as a structure prop and also as an ionic binding site for the acid dyes during overprinting. Additional ionic dye sites are supplied by the diamine salt, which further improves shade depth in the overprinted material.
A number of advantages accrue from this method:
The fibers already have a base or background color prior to overprinting. This eliminates the need to dye the fabric prior to printing or eliminates one color in the printing process.
The fibers are substantive to acid dyes (the only dyes acceptable for some camouflage applications because of IR reflectance). Aramids have a substantivity for basic (cationic) dyes, but not acid dyes.
There is no need for carriers or solvents to swell the aramid structure to allow dye penetration. This can prove to be a difficult, and time-consuming, step when incorporating a dye or a dye site substance into aramid fibers which have been previously dried during processing.
A high temperature roll heating step is not required to remove solvents used in the dyeing process.
A U.V. screener can be included in the fibers, without any additional steps, to improve lightfastness.
A conventional rotary screen printing process may be used to print the fibers, or fabric made from such fibers.